Electric fuse



Aug. 5, 1941. F. s. KLEIN ELECTRIC FUSE Filed May 15, 1939 INVENTOR.

Patented Aug. 5, 1941 UNITED STATES PATENT OFFICE ELECTRIC FUSE FranlninS. Klein, Malverne, N. Y.

Application May 15, 1939, Serial No. 273,882

Claims.

This invention relates to electric fuses of the type known as delayedaction or time lag fuses which are designed to provide the necessaryprotection to the components of an electric circuit without blowingneedlessly; and like such fuses, my new fuse can be overloaded withseveral times its rated capacity for a sufficient length of time toallow the passage of a harmless transient overload of current such asthe unavoidable heavy current for starting an alternating current motor;but the same fuse will open the circuit when a relatively light overloadtends to continue for such a length of time as would be harmful; or blowpromptly when a short circuit or dangerous overload occurs. Therebyproviding all the protection necessary while avoiding a needlessdisruption of the fuse.

In each of the prior delayed action fuses that I know of, there is oneor more of several objectionable features which are avoided in myimproved fuse; for instance, strain 01' tension imposed on a fusibleelement by means of weight or a spring, causes a disruption of thefusible element before the metal thereof is actually melted by anoverload of current; and to compensate for this fault, said tension isreduced, or the body of fusible metal increased or fusible metal ofhigher melting point is used, but such expedients are not favorable tosafety.

Some delayed action fuses include a high resistance element wherepurposely, the current generates heat, which, when the fuse isoverloaded, is sufficient to melt a fusible element and open thecircuit. In one of these fuses the high resistance element is in theform of a coiled wire; in another, graphite; in another, granulatedcarbon; and in another, a carbon rod. These are all expensive adjunctsto a fuse and are further objectionable in that resistance elementscause perceptible heat and a Waste of current, especially when operatednear the rated capacity of the fuse.

In some of these prior fuses, tension is necessarily applied to afusible element to help open the circuit and stop the electric are bymeans of a springy device, usually of metal which eventually losesnecessary tension because of frequent heating by a resistance element,thereby unfavorably increasing the expected time lag, or volatilizationwhen the link is opened, i. e., electrically disrupted; and in manyinstances it is evident that the arc operates more swiftly than theelement under tension and consumes much metal; moreover, elements fortension add to the cost of a fuse.

In my improved fuse, it will be seen that, a destructive arc iscurtailed, a delayed action provided, tamperproofing provided, and theaforementioned objectionable features of prior fuses avoided, by meanswhich, in comparison with prior fuses, are so economical and effectiveas to be important to both manufacturers and consumers.

In prior applications bearing the Serial No. 273,883 filed May 15, 1939,and substituted for an application bearing Serial No. 157,288 filedAugust 4, 1937, and in the copending application bearing Serial No.206,473 filed May 6, 1938, I have described fuses having arc blocking,time lag, and tamperproof features; these are to be found in this laterfuse of which the principal object is to provide a delayed action ortime lag fuse which will cost less, and function better and longer thanprior fuses of the same type.

In attaining my objects, I use a basic fuse link designed to carry atemporary and relatively heavy overload, in other words a transientoverload of possibly as much as eight times the normal capacity of thecircuit served, and then only for a safe length of time which variesaccording to the magnitude of the current; obviously this allows variousharmless transient heavy overloads without disrupting or opening thefuse link unnecessarily; and in the middle of that section of the linkwhich is most susceptible to heat and the initiation of volatilization,I sever the link and provide two of such sections having ends, and thenreconnect or bridge these severed ends with a body of metal which isadapted to carry a predetermined and greater current than either of saidtwo sections and be displaced by the heat caused in said two sections bya relatively light overload which may delay action in said two sectionsand cause damage in the circuit.

Then I enclose the fuse link in a suitable casing composed ofnonelectric conducting or dielectric material; and in combination withthe casing and the fuse link described above, I use a material,preferably in the form of cement, which is dielectric, flameproof, heatconducting, and refractory or unmanageable; and arrange that material sothat it adheres to the fuse casing, so as to form, substantially, a partof the dielectric material of the casing, and with the casing formanunmanageable mass which includes a substantial portion of at least oneof the aforementioned fusible sections of the link, but preferably themajor part of the whole link including both ends of the link. So as toleave only a relatively short portion of the fusible section or sectionsof the link extending from said mass with the aforementioned body of lowmelting point metal a relatively short distance from the mass. Whereby,the mass will conduct heat from the fuse link and delay the melting ofthe body of low melting point metal, and limit a disruption to arelatively short portion of the whole link by blocking an arc close tothe beginning of a rupture in the link, and furthermore, preventtampering with the value of the fuse link before or after a rupture ofthe link.

In the accompanying drawing, Fig. 1 is a sectional view of my improvedcartridge fuse; Fig. 2 is a sectional view of a cartridge fuse includingfurther improvements; Figs. 3 and 4 are sectional views of improvedscrew plug fuses; Fig. 5 is an enlarged view of the fusible element ofthe'new fuse.

In further detail, and in each figure of the drawing, a dielectriccasing is shown at the number I, having contact terminals at 2 and 3,and a fuse link at i, which is severed, that is, divided, at 5 the placemost susceptible to heat and a disruption, which is usually, but notalways, the middle of the link where the least heat is absorbed by otherportions such as heavier end portions or terminal connections.

Where the link is severed a small section of the metal is removed toprovide a definite space as at 5 between the two severed ends 8 and 9which are bridged with metal if) having a lower melting point than thepotentially fusible metal in the link l; this bridge or joint at H] isclearly shown on an enlarged scale in Fig. 5.

This new fuse may include any of the different metals or alloys,peculiar advantages are to be found in lately discovered alloys, but forexample, I will assume the link at 4 to be of copper for I shall usecopper in many instances because it serves well to set forth myinvention, and copper is relatively stable and slow to melt at hightemperature, it has high conductivity for heat and current, the meltingpoint is 1065 C. and less metal need be used and volatilized; all ofthis contrasts well with the lower melting point of many metals whichmay be used to form a bridge at it which must be relatively less stableat high temperature than the basic link 4.

The bridge at it may be made of zinc, tin, lead, or any alloy having thedesired melting point, and may possibly be a mixture of 32% lead, 50%tin, and 18% cadmium and known to melt at 150 C.; of course the time lagdesired determines which metals are to be used, for, while some machineswithstand quite heavy overloads, delicate devices need a sensitive fuse.

Since a metal having a relatively low melting point has relativelygreater resistance to current, the bridge I ii must have sufficientcross-sectional area to provide greater conductivity for current thanthe basic link l; and as shown, the metal at 19 is to be soldered asthat is the best connection for the conduction of heat into the bridge.

The body of metal forming the bridge i is to have such characteristicsand to be so proportioned, with respect to the basic link 4, that theweight or electrical resistance of the bridge will not be a factor ofits displacement before the melting point of the bridge is reached.

With reasonable care the proper metal can be selected to make the bridgeW of little weight and smaller size than those bridges shown in thedrawing, which perforce must illustrate details; an object, is topresent the least'm'etal to feed an electric arc.

In fuses of low current rating, the weight of the bridge at H] may notbe suificient to displace the bridge when melted, nevertheless, it willbe displaced by volatilization because of the increased resistance ofthe bridge metal when in a melted state.

In a fuse link having a bridge as described above, various heavy butharmless temporary surges of current, will not melt the relativelystable metal in the basic link 4 or heat that link sufiiciently to meltthe bridge [U which has the capacity to carry such overloads, andbecause such overloads are so often normal and expected in the electriccircuit served by a fuse link, they are hereinafter termed normaltemporary overloads, but when a relatively light overload tends tobecome permanent and harmful, in other words an abnormal overload, heatwill gradually increase in the two potentially fusible sections II andII and melt and displace the lower melting point metal of the bridge l0;also a. dangerous overload or short circuit will disrupt such a link atthe sections H and H as promptly as it would disrupt any other fusewhich is rated for the normal current in the circuit served.

Fig. 1 shows a fuse of the cartridge type which includes a tubularcasing I, metal ferrules and blades 2 and 3 for contact terminals whichmay be of any other type desired and may also be removable to renew thefuse link; and the fuse link at 4 is the same as the new link describedin detail above.

Fig. 2 also shows a cartridge fuse having a casing and terminals asdescribed in Fig. 1 but in this cartridge I follow a common practice anduse a fuse link having end portions 6 and I which are relatively longportions of the link and together constitute the major part of the wholelink, also an intermediate longitudinal section at H which is reduced incross-section and a relatively short and particularly fusible section ofthe link which will carry more current than the same cross-sectionthroughout a fuse link, and thereby curtail the metal volatilized andlocalize a disruption; this link is also severed at 5 to provide two ofsaid short sections as at H and II having ends 8 and 9 which are bridgedat 10 as shown and described in Figs. 1 and 5.

In Fig. 2, at I2 is an arc blocln'ng device which will minimize adisruption in the fuse and its violence. This arrangement issubstantially the same as described in the aforementioned applicationNo. 206,473, and likewise, this drawing shows at l2 an unmanageable massof cement Which is dielectric, flameproof, and contains particles ofporcelain or other material which will conduct heat from the fuse link.

The are blocking device at i2 is a dielectric material arranged to forma cross-section of the casing i into a solid, and considerable masssafely embedding a substantial portion of the particularly fusiblesection II to prevent destruction of the same by an arc, and leaving theremainder of that fusible section H and the bridge in, to extend fromand clear of said mass to disrupt freely either at the bridge ill or atsaid sections II and H according to the magnitudeof an overload; andwhatever the overload may be, the disruption will be limited to thefusible sections H and H which, including the bridge l3, constitute avery short portion of this link.

While I have specified a cross-section of a fuse casing arranged to forma solid and considerable mass I trust that I am not to be restricted tothe same, for I contemplate other means for embedding portions of thefuse link as described, without filling a full cross-section of thecasing.

At Fig. 2 is shown an economical arrangement of the are blocking massl2; which is a thin washer E4 of incombustible material threaded tightlyover the link A and forming a stop at the proper place on the fusiblesection II for a cement 12 which is poured into the combustion chamberI5 and embeds part of said section H; in renewable fuse cases this arcblocking device may be of any other convenient arrangement to permitreplacements.

At Fig. 3 my invention is included in a fuse of the plug type, and Ihave chosen to show a fuse having a dielectric casing I, an annularscrew shell contact terminal 2, a bottom contact terminal 3, acombustion chamber Ill having a bottom end closed at i6, and a top partincluding a cap ll having a window I8 of transparent material, and afuse link 4 which, in detail, is the same as other links 4 with bridgesI described above, standing in the combustion chamber l so as to form aloop which extends toward said top part i! of the chamber IS with thebridge l0 and fusible sections I! and H forming the upper part of theloop.

Fig. 3 includes an are blocking device substantially the same as thatdescribed in Fig. 2 but in this instance the cement [2 comprising thisdevice, is poured into the bottom end or lower cross-section of thecombustion chamber in sufficient mass to safely embed the major portionof each end of the fuse link and end parts of each fusible section IIand II in are blocking material, which preferably is to be of arefractory character, leaving the bridge ill and its adjacent portionsof the fusible sections H and H forming a relatively small loop whichstands above and clear of the cement.

In the fuse at Fig. 3 a disruption will be brief and limited to arelatively small part of the link and those portions of the link whichare embedded in the cement l2 are tamperproof and to reestablish acircuit through this casing, is impractical. At the same time thiscement materially extends any delay of action that may otherwise beexpected in a fuse link.

At Fig. t is shown a fuse plug the details of which are the same asdescribed at Fig. 3 above, excepting that, in this fuse the fuse link iscontinuous, there is no intermediate joint or bridge or other metal;nevertheless it is a novel fuse in that only a small portion of thewhole fuse link is looped to stand clear of the are blocking body ofmaterial at l2.

At Fig. 4 the are blocking device at 12 is to be made of a mixturecontaining a large proportion of porcelain or sand to absorb heat fromthe fuse link and delay the action of a relatively light and temporaryoverload of current, and furthermore a disruption in this fuse, whennecessary, will be free to occur and brief, in other words; there isnothing to prevent a prompt opening of this link in clear and sufficientspace for the dispersion of such gas as may occur here, and the durationof the accompanying are or flow of current through gas will be limitedby a relatively small amount of metal being available to an arc, andthat metal may be considerably less than that section of the link whichI have shown to be extending clear of the are blocking material at I2 toillustrate the advantages of such an arrangement.

Having described my invention I claim:

1. An enclosed electric fuse having a casing with contact terminals anda fuse link of the delay action type comprising a fusible strip dividedinto two sections, and having one end of each section bridged togetherby a relatively short body of metal having a greater current carryingcapacity and a lower melting point than said sections, said sectionshaving the capacity to carry a normal temporary overload in an electriccircuit and be heated by an abnormal overload and thereby displace saidbody of metal, and also be fused by a short circuit in said electriccircuit; in combination with said casing and fuse link, a body ofmaterial which is heat conducting, fiameproof, unmanageable, andarranged to form an unmanageable mass which includes a portion of saidcasing and a substantial portion of at least one of said fusiblesections, a relatively short portion of a fusible section of said stripand said body of metal extending clear of said mass whereby, said massmay conduct heat directly from the fusible section to delay the meltingof said body of metal, and block an electric are close to the beginningof a rupture in the link, and prevent tampering with the value of saidfuse link.

2. In an electric fuse of the plug type having a casing, contactterminals, a combustion chamher in the casing, and a fuse link in thecombustion chamber forming a loop; the combination therewith of a bodyof material which is dielectric, flameproof, heat conducting,unmanageable, and arranged in part of said combustion chamber to form anunmanageable mass which includes a section of said casing and the majorpart of said link including both its ends and leaving a relatively shortportion of the whole link extending in a loop clear of said mass,whereby,, said mass may conduct heat from the major: part of said linkand delay a rupture of said short portion of the link, cause an abnormaloverload to open the circuit promptly at said portion of the link, andprevent tampering with the value of said link.

3. An electric fuse of the screw plug type having a dielectric casing,contact terminals, and a fusible strip having a relatively short sectionthereof reduced in cross-section, located intermediate less fusiblelongitudinal portions of the strip and forming a loop toward the top ofthe casing; in combination with a body of cement which is heatconducting, flameproof, unmanageable, and arranged to form the lowercrosssection of said casing and the major part of said strip into anunmanageable mass, the remainder of said strip being only the topportion of said loop extending above said mass whereby, said mass mayconduct heat from the fusible strip and delay a disruption thereof, andlimit a rupture of said strip to said top portion of the loop byblocking an electric arc close to the rupture, and also preventtampering with the value of said fusible strip.

4. An electric fuse of the screw plug type having a dielectric casing,\contact terminals, a Window composed of transparent material, and fuselink having two longitudinal fusible sections connected in series by abody of metal having a greater current carrying capacity and a lowermelting point than said sections, said sections forming a loop with saidbody of metal toward said window, said sections having the capacity tocarry a normal temporary overload in an electric circuit, and be heatedby an abnormal overload and thereby melt said body of metal,

and also be fused by a short circuit in said electric circuit; incombination with, a body of material which is heat conducting,flameproof, dielectric, unmanageable, and arranged to formanunmanageable mass which includes a crosssection of said casing and themajor part of said fuse link, the remainder of said link being said bodyof metal and a portion of each of said fusible sections forming arelatively small loop extending above said mass whereby, said mass mayconduct heat from the fusible sections and delay the fusing of said bodyof metal, and limit a rupture of the fuse link to said relatively smallloop by blocking an electric are close to the'loop', and also preventtampering with the value of the fuse link.

"5; An enclosed electric fuse having a casing, contact terminals, and afuse link having a relatively short section of its length adapted todelay the action of a normal temporary overload of current through thelink and also adapted to be opened by an abnormal overload of current;in combination with, a body of material which is heat conducting,fiameproof, unmanageable, and arranged to form an unmanageable masswhich includes a substantial cross-section of said casing and the majorpart of said fuse link, with a portion of said relatively short sectionof the length of the link extending clear of said mass whereby, saidmass may further delay the action of an overload of current byconducting heat directly from said relatively short section of the fuselink, and limit a rupture of the link to said relatively short portionextending from the mass by blocking an electric are close to therupture, and also prevent tampering with the value of the fuse link.

FRANKLIN S. KLEIN.

